Protein composition and process for isolating a protein composition from a muscle source

ABSTRACT

A process is provided for isolating a protein component of animal muscle tissue by mixing a particulate form of the tissue with an acidic aqueous liquid having a pH below about 3.5 to produce a protein rich solution substantially free of myofibrils and sarcomere tissue structure. The protein rich aqueous solution can be treated to effect protein precipitation, followed by protein recovery.

REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of application Ser.No. 08/797,929 filed Feb. 12, 1997, which in turn, is acontinuation-in-part of provisional application Serial No. 60/034,351,filed Dec. 21, 1996.

This invention was made with government support under GrantNA90AA-D-SG24 awarded by the U.S. Department of Commerce (NOAA).

[0002] The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] This invention relates to a process for recovering protein froman animal muscle source with improved functional properties and to theprotein product so-obtained. More particularly, this invention relatesto a process for recovering muscle proteins with improved functionalproperties from an animal source and the protein product so-obtained.

[0005] 2. Description of Prior Art

[0006] Presently, there is an interest in expanding the use of muscleproteins as food because of their functional and nutritional properties.Better use of these materials would be particularly important with agedor frozen raw materials which are less valuable because they have lostprotein functionality. It is presently believed that the muscle tissueutilized as the feed in present processes must be fresh rather thanfrozen or aged. It is common commercial practice to process freshlycaught fish at sea on board ship rather than subject the fish to thetime of transportation or the freezing necessary to effect processing onland. Ageing or freezing of fish lowers the functional qualities of thetissue proteins. Protein functionalities of most concern to foodscientists are solubility, water holding capacity, gelation, fat bindingability, foam stabilization and emulsification properties.

[0007] Protein concentrates from muscle tissue, especially fish, havebeen made by hydrolysis. This approach has improved some functionalproperties, particularly solubility, which has allowed its use inprepared soups. However, this approach also destroys other functionalproperties such as gelling ability.

[0008] One process that has had some success in stabilizing proteinfoods has been the process for producing “surimi”. This conventionalprocess has been used primarily for fish, although there have been someattempts to produce a surimi-like product from other raw materials suchas mechanically deboned poultry mince. In producing surimi, the freshmuscle is ground and washed with a variable amount of water a variablenumber of times. This is determined by the location of the plant and theproduct that is desired from the particular species. Water may be usedin a ratio as low as about 2 parts water to one part fish up to about 5parts water per 1 part fish; typically about 3 parts water are used per1 part fish. The number of washes can vary, generally, from 2 to 5,again depending on the raw material, the product desired, and wateravailability. Twenty to thirty per cent of the fish muscle proteins aresolubilized when the ground muscle is washed with water. These solubleproteins, known as sarcoplasmic proteins, are generally not recoveredfrom the wash water of the process. This loss is undesirable sincesarcoplasmic proteins are useful as food. The washed minced productcontaining the protein in solid form then is used to make protein gels.Originally, this was used to produce “kamaboko” in Japan. Kamaboko is apopular fish sausage in which the washed minced fish is heated until itgels. It is presently believed that it is necessary to addcryprotectants to the washed, minced fish before freezing to preventprotein denaturation. A typical cryoprotectant mixture comprises about4% sucrose, about 4% sorbitol and about 0.2% sodium tripolyphosphate.These components retard the denaturation of the protein during freezing,frozen storage and thawing.

[0009] It has been proposed by Cuq et al, Journal of Food Science, pgs.1369-1374 (1995) to provide edible packaging film based upon fishmyofibrillar proteins. In the process for making the films, the proteinof water-washed fish mince is solubilized in an aqueous acetic acidsolution at pH 3.0 to a final concentration of 2% protein. No attemptwas made in this work to re-adjust the pH values of the acidifiedproteins to re-establish the functional properties attained at pH valuesabove about 5.5. In addition, the use of acetic acid imparts a strongodor to the material which would severely limit its use in a foodproduct.

[0010] It also has been proposed by Shahidi and Onodenalore, FoodChemistry, 53 (1995) 51-54 to subject deboned, whole capelin to washingin water followed by washing in 0.5% sodium chloride, followed bywashing in sodium bicarbonate. The series of washes, including thatusing sodium bicarbonate, would remove greater than 50% of the muscleproteins. Essentially all of the sarcoplasmic proteins would be removed.Final residue was further washed to remove residual bicarbonate. Thewashed meat was then suspended in cold water and heated at 70° C. for 15min. This heat treatment is sufficient to “cook” the fish proteins, thusdenaturing them and reducing or eliminating their functional properties.No attempt was made to restore proteins to improve the functionalproperties of the capelin proteins.

[0011] Shahidi and Venugopal, Journal of Agricultural and Food Chemistry42 (1994) 1440-1448 disclose a process for subjecting Atlantic herringto washing in water followed by washing with aqueous sodium bicarbonate.Again, this process will remove greater than 50% of the muscle proteins,including the sarcoplasmic proteins. The washed meat was homogenized andthe pH varied between 3.5 and 4.0 with acetic acid. In addition, thereis an unacceptable odor problem with the volatile acetic acid.

[0012] Venugopal and Shahidi, Journal of Food Science, 59, 2 (1994)265-268, 276 also disclose a similar process for treating mincedAtlantic mackerel. The material is washed sequentially with water,bicarbonate solution and again water. The pH is brought to pH 3.5 withacetic acid after homogenization. The proteins were precipitated at pHvalues greater than 4 on heating the material to 100° C. for 15 min. Itis disclosed that “dissolution of structural proteins of fish musclerequires extractants with an ionic strength >0.3”.

[0013] Shahidi and Venugopal, Meat Focus International, October 1993,pgs 443-445 disclose a process for forming homogenized herring, mackereldispersions or capelin dispersions in aqueous liquids having a pH as lowas about 3.0. It is reported that acetic acid reduces the viscosity ofherring dispersions, increases viscosity of mackerel to form a gel andprecipitates capelin. All of these preparations were initially washedwith water and sodium bicarbonate, which would remove a substantialproportion of the protein, including the sarcoplasmic proteins.

[0014] Chawla et al, Journal of Food science, Vol. 61, No.2, pgs362-366, 1996 discloses a process for treating minced threadfin breammuscle after it has been washed twice with water and recovered byfiltration. The minced fish product is mixed with tartaric, lactic,acetic or citric acid, is allowed to set and then is heated in a boilingwater bath for twenty minutes and then cooled to form a gel. This heattreatment is sufficient to denature the proteins. The washing stepsundesirably remove soluble sarcoplasmic proteins from the mince. It isalso disclosed that unwashed mince failed to provide the desired gelforming property of surimi.

[0015] Onodenalore et al, Journal of Aquatic Food Products Technology,Vol. 5(4), pages 43-59 discloses that minced shark muscle is a source ofacidified protein compositions. The minced product is washedsequentially with aqueous sodium chloride, aqueous sodium bicarbonateand then water to remove metabolic substances. This washing effectsundesirable removal of sarcoplasmic proteins. The minced product isrecovered by filtration. The minced product then is acidified to pH 3.5with acetic acid, heated in a boiling water bath, cooled and centrifugedto recover a supernatant. The supernatant pH was adjusted to a pH 4-10using NaOH, heated in a boiling water bath, cooked and centrifuged torecover a second supernatant. Heating the protein dispersion comprisingthe minced product resulted in 87-94% of the protein remaining insolution while heating of the unacidified protein dispersion resulted inprotein coagulation. However, the heating causes protein denaturation.

[0016] Accordingly, it would be desirable to provide a process forrecovering a high proportion of available muscle protein from an animalsource including a frozen or aged animal source, rather than requiring afresh muscle tissue source. It would also be desirable to provide such aprocess, which permits the use of muscle protein sources which arepresently under-utilized as a food source such as frozen or aged fish.Furthermore, it would be desirable to provide such a process whichrecovers substantially all of the protein content of the process feedmaterial. In addition, it would be desirable to provide such a processwhich produces a stable, functional, protein product which isparticularly useful for human consumption. Such a process would permitits operation at will rather than require initiation of the process veryshortly after the animal source is killed so that processing can beextended over a desired time schedule.

BRIEF DESCRIPTION OF THE INVENTION

[0017] This invention is based upon our newly discovered properties ofthe myofibrillar and sarcoplasmic proteins of muscle tissue which permittheir processing at low pH, below about 3.5. Muscle tissue (fish ormeat) is disrupted to form particles, such as by being ground orhomogenized with enough water and at a pH to solubilize a majorproportion, preferably substantially all of the available protein.Solubilization is effected at a low pH below about 3.5, but not so lowas to effect substantial destruction of proteins, preferably betweenabout 2.5 and about 3.5. During the solubilization step, the myofibriland sarcomere tissue structure is substantially completely converted tosolubilized protein so that the final product obtained as describedbelow is substantially free of the myofibril and sarcomere tissuestructure. This process differs from the conventional process for makingsurimi in that major myofibrillar proteins are never solubilized in theconventional process. In the conventional process, of making surimimyofibrillar proteins are simply washed in water or in water that hasbeen made slightly alkaline to remove water-soluble materials that leadto loss of quality of the product. Unfortunately, this conventionalprocess also removes water-soluble sarcoplasmic proteins.

[0018] In an optional embodiment of this invention, the disrupted muscleissue can be mixed with an aqueous solution to give a pH typicallybetween about 5.0 and about 5.5 to provide a suspension of muscleparticles which can be more easily treated to solubilize proteins in thesubsequent low pH treatment step to produce a solution having asufficient low viscosity, i.e, a non-gel, so that it can be easilyprocessed. By conducting this optional preliminary step at pH betweenabout 5.0 and about 5.5, a homogeneous suspension is obtained whereinthe protein does not imbibe excessive concentration of water. Thus,reduced volumes of water are processed which must be treated to effectthe desired lower pH in the subsequent solubilization step.

[0019] The solubilized protein material from the low pH treatment step,then is treated to precipitate the proteins such as by raising its pH tobetween about 5.0 and about 5.5, addition of salt, the combination ofsalt addition and increase in pH, the use of a coprecipitant such as apolysaccharide polymer or the like to recover an insoluble proteinproduct containing myofibrillar proteins and a significant proportion ofthe sarcoplasmic protein of the original muscle tissue proteins in theoriginal muscle tissue process feed. The protein product can containmembrane protein present in the original animal tissue process feed.Also, as set forth above, the precipitated protein is substantially freeof myofibril and sarcomere tissue structure. Myofibrils and sarcomeretissue comprise strands of tissue or portions of tissue strand structurewhich can be viewed under a microscope. Myofibrils and sarcomere areformed primarily of proteins.

[0020] In an alternative process of this invention, the muscle tissuecan be washed to obtain an aqueous solution of sarcoplasmic protein.This solution is treated at low pH as set forth above and thenprecipitated as set forth above in the presence of myofibrillar protein.

[0021] In an alternative process, this precipitation step need not beconducted to recover the protein product. The protein product can betreated directly without raising its pH such as by precipitation with asalt, polymer or the like and can be spray dried to be used, forexample, in acidic foods. Alternatively, the low pH protein-richsolution can be treated to improve its functional properties, such aswith an acidic proteolytic enzyme composition or by fractionating theprotein.

[0022] The precipitated protein composition recovered at the higher pHcondition can be further treated to produce a food product. Such furthertreatment can include lyophilization, freezing with or without an addedcryoprotectant composition and with or without raising its pH orgelation by raising its pH.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a general schematic diagram illustrating the process ofthis invention.

[0024]FIG. 2 is a schematic diagram of a conventional process of theprior art.

[0025]FIG. 3 is a schematic view of an improved conventional process ofthe prior art.

[0026]FIG. 4 is a schematic view of a preferred process of thisinvention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

[0027] In accordance with this invention, animal muscle is disrupted toform particles such as by grinding, homogenization or the like. As anoptional preliminary step, the animal muscle tissue source of protein isground and mixed with an aqueous liquid at a pH below about 3.5 and at aratio of volume of aqueous liquid to weight of tissue to form an aqueouscomposition which does not have an undesirably high viscosity whichrenders recovery of the protein difficult. As a result of this low pHcondition, the protein solution is rendered substantially free ofmyofibrils and sarcomeres. The animal muscle source can be fresh, agedor frozen. Typically, the ratio of volume of aqueous liquid to weight oftissue is greater than about 7:1, preferably greater than about 9:1.Lower ratios of volume of aqueous liquid to tissue weight can beutilized, depending upon the species source of muscle tissue when theanimal muscle source exhibits a reduced tendency to cause gelation atthe lower ratios. By utilizing these conditions of pH and ratio ofaqueous liquid volume to tissue weight, the protein component of thetissue is dissolved in the aqueous liquid while avoiding gelation of thecomposition in this step. The pH should not be so low as to destroy asubstantial portion of the protein over the time period the protein isin solution i.e., below about pH 1.0. Protein denaturation and proteinhydrolysis also are a function of temperature and time in solution withincreased temperature and increased time in solution promoting proteindenaturation and protein hydrolysis. Thus, it is desirable to reducesolution temperature and the time protein is in solution, particularlywhen a lower pH of the protein solution is attained, for example about2.0 or below. The aqueous liquid composition also may contain componentswhich do not degrade or hydrolyze the proteins in solution such assalts, for example, sodium chloride or the like. The ionic strength ofthe solution should be maintained below about 200 mM to avoid proteinprecipitation when it is not desired.

[0028] In an optional preliminary step, the disrupted animal muscletissue is mixed with an acidic aqueous solution to a pH of about 5.0 toabout 5.5. Thereafter, the pH of the mixture is reduced with acid asdescribed above in order to solubilize the proteins. It has been foundthat this preliminary mixing step provides protein solutions of reducedviscosity in the low pH treatment step described above and therefore,promotes ease of processing to recover the protein.

[0029] At this point, the solubilized composition optionally can befractionated, in order to recover a particular desired protein fractionor derived product fraction if desired by size exclusion chromatographyor other techniques based on properties of the proteins, other thanmolecular size, since the materials are solubilized in a solution of lowviscosity. Alternatively, the protein in solution can be dehydrated, forexample, by spray drying, to produce a functional protein for use inacid foods such as salad dressing, mayonnaise, gels or as a nutrientsupplement to fruit juices, sodas, or the like. This point of theprocess provides a convenient time to treat the dissolved proteins withacidic proteolytic enzymes, if desired to modify the proteins to improvetheir functional properties as desired. Some limited proteolysis mayoccur at the low pH. This proteolysis depends on time, temperature, andthe specific pH value.

[0030] The recovered protein-rich solution/colloidal suspension can thenbe adjusted to a pH at which essentially all of the proteinsprecipitate, such as between about 5.0 and about 6.5. This pH will varydepending upon the animal source of the protein and is generally betweenabout 5.0 and about 5.5, more usually between about 5.3 and about 5.5.The protein can be recovered again, such as by centrifugation or with apolymeric precipitant, e.g., a polysaccharide or combination thereof orthe like. Not only are all of the myofibrillar and cytoskeletal proteinsrecovered, but the soluble sarcoplasmic protein fraction which has beenpreviously solubilized at the reduced pH below about 3.5, but notseparately recovered is also precipitated by raising the pH to betweenabout 5.0 and about 5.5. This recovery of the sarcoplasmic proteins isnot observed when the sample is directly reduced in pH to about 5.5 andcentrifuged. It is necessary to attain the low pH condition and thenreturn to the pH condition where protein precipitation is effected toprevent this protein loss. When the low pH condition is notpreliminarily obtained, the protein loss is generally between about 20and about 30% of the original process feed protein, primarily due toloss of sarcoplasmic protein. The precipitated protein is separated fromthe aqueous liquid compositions which contain soluble impurities such aslow molecular weight metabolites, sugars, phosphates and/or nucleotides.Alternatively, protein precipitation can be attained with precipitatingpolymers such as polysaccharides, charged polymers, marine hydrocolloidsincluding alginates or carrageenan or the like either alone or incombination with centrifugation. In addition, as set forth above,precipitation can be effected by addition of salt or by the combinationof pH control and addition of salt. While applicants do not intend to bebound by a particular theory to support unproved protein recovery, thisenhanced recovery may be due to either molecular changes in thesarcoplasmic proteins whence they become insoluble at that pH, or theymay bind more readily to the myofibrillar and cytoskeletal proteins dueto molecular changes in the latter proteins. Alternatively, it may bethat the opening up of the myofibrillar and cytoskeletal proteinsprovide more binding sites for the sarcoplasmic proteins.

[0031] In any event, applicants have found that treating the proteinsolution at the low pH condition set forth above improves thefunctionality of the protein. This observed improvement permits the useof aged or frozen muscle tissue as a starting material in the process ofthis invention. In addition, fresh muscle tissue can be used as astarting material in the process of this invention.

[0032] The rate at which the pH of optimal precipitation is reached canhave an effect on the nature of the association of the collectedproteins. Rapid change in pH by direct addition of base can produce anaggregated mass of proteins whereas a slow change in pH, for example,that achieved by dialysis, can allow proteins to specifically associatewith the proteins with which they are normally associated in thefibrils.

[0033] Any acid that does not undesirably contaminate the final productcan be used to lower the pH such as organic acids including citric acid,malic acid, tartaric acid or the like or mineral acids such ashydrochloric acid or sulfuric acid or the like or mixtures thereof.Citric acid which has favorable pKa values is a preferred acid for theprocess. Sufficient citric acid provides adequate buffering capacity atpH 3 and pH 5.5 and then hydrochloric acid can be used to reduce the pHto the desired point. Acids that have significant volatility whichimpart undesirable odor such as acetic acid or butyric acid areundesirable. Likewise, any of several bases can be used to raise the pH.It is preferred to add a polyphosphate since this also functions as anantioxidant and improves the functional properties of the muscleproteins.

[0034] The precipitated protein optionally can be treated in many ways.For example, its pH may be raised to neutrality, cryoprotectants added,and frozen to make a typical “surimi”. Surimis prepared by this processhave excellent functional quality. The “true strain” (a measure ofprotein quality) has been as high as 2.8 for cod and 2.6 for lightmuscle as animal protein sources. The product has reduced lipid. Theprecipitated protein can be dehydrated after the addition of agentscurrently used in surimi processing such as starches to preventaggregation of the protein, such as, but not limited to, negativelycharged compounds for use in the production of products such as gels,emulsifiers and viscosity developers. The precipitated protein can alsobe re-acidified to pH of from about 2.5 to about 3.5 using less liquidvolume than it previously contained to concentrate the protein prior todehydration. This provides energy savings for the dehydration step. Inaddition the recovered protein compositions can be fractionated torecover constituent proteins. The resultant product is useful as aningredient in products such as those described above.

[0035] When utilizing animal muscle tissue which is relatively high infat, oil and/or lipids, the fat, oil and/or lipid can remain with theprecipitated protein which may render the protein-rich productsusceptible to degradation, primarily by oxidation. Accordingly, theprotein-rich product can be treated such as by being stored in a vacuum,stored frozen, by adding an antioxidant thereto such as isoascorbicacid, ascorbic acid, erythorbic acid, propyl gallate, tocopherols or thelike.

[0036] This invention improves upon the prior art in that:

[0037] 1. Aged or frozen muscle tissue can be used as a feed compositionwhich permits scheduling of the process to accommodate a desired timeperiod. It is not necessary in the process of this invention to requirevery fresh product as a starting material. The ability of the process ofthis invention to use non-fresh and even frozen fish is very importantfor a fishing fleet catching the fish and permits use of shore-basedplants to effect the process of this invention since it eliminates therequirement for using fresh fish fillet sources now required bypresently available processes.

[0038] 2. The process of this invention provides for increased yield ofprotein. Greater than about 90% of protein typically are obtained fromlight muscle tissues with the process of this invention, whereas priorart similar processes provide less than about 60% protein recovery. Insome cases, the protein yields obtained with the present invention areas great as about 95%.

[0039] 3. The improved yield of protein as product means that there isless protein to recover/remove in the waste water, so that by-productpollution is decreased.

[0040] 4. The color of the product of this invention is much improvedover the color of prior art products. The color of surimi now made frompelagic fish with presently available processes is typically grayish incolor with a high Hunter “b” value. A white color as good or better thanthe best grade of surimi made from lean white-fleshed fish frompresently available processes is obtained with the process of thepresent invention from the light muscle of mackerel as the startinganimal protein source. As a process feed material, mackerel light musclefrom fish stored between 2 and 3 days on ice typically provides aproduct of this invention having “L”, “a”, “b” values of 78.4, −0.89 and2.0 with a whiteness index of 78.3 or better.

[0041] 5. In prior art processes, a majority of the muscle proteins areinsoluble throughout the process. The process of this inventionsolubilizes approximately 98% of the muscle proteins. This reducesprocess time, promotes process ease of control and renders the processadaptable to continuous processing.

[0042] 6. An obvious use for the process of this invention is to utilizematerials which are not available now as human foods because of theirinstability and unfavorable sensory qualities. Stability can be improvedwith the process of this invention when utilizing stability enhancingcompositions such as antioxidants or the like. An example of the use inthe present invention are the small pelagic species of fish such asherring, mackerel, menhaden, capelin, anchovies, sardines, or the likeas starting materials, which presently are either underutilized or areused primarily as industrial fish and not for human consumption.Approximately one half the fish presently caught in the world are notused for human food. A process that produces an acceptable stableprotein concentrate for human consumption constitutes an importantvalue-added use of this material and an important contribution to worldnutrition. For example, the estimated annual sustainable yield ofmackerel, menhaden and herring available off the Atlantic coast of theUnited States is as high as 5 billion pounds. The process of thisinvention also can be used to process flesh that is recovered fromfarmed fish after the fillets have been removed. This material currentlyis not used for human food. Representative suitable starting sources ofanimal protein for the process of this invention include fish fillets,deheaded and gutted fish, including pelagic fish, crustacea, e.g.,krill, mollusc, e.g. squid or chicken, beef, lamb, sheep or the like.For example, a large quantity of mechanically deboned chicken meatpresently is produced from the skeletons of the birds after chickenparts are removed for retail sale. The process of the present inventioncan utilize such chicken parts to produce protein rich product usefulfor human enterprise. Other under-utilized muscle sources adaptable tothe process of this invention include Antarctic krill, which isavailable in large quantities but is difficult to convert to human foodbecause of its small size. The process also is capable of utilizing mostunstable or low value muscle tissue.

[0043] A specific example of the process of the present inventioncomprises a plurality of steps, including optional steps. In a firststep, an animal protein source is ground to produce a composition ofparticles having a high surface area which promotes subsequentprocessing. In an optional second step, the ground protein source can bewashed with water, typically with about 1 to 9 or more volumes of waterbased on the weight of ground muscle source. When utilizing the optionalwashing step, the liquid soluble fraction is separated from theinsoluble fraction such as by centrifugation with the insoluble fractionbeing processed further as described below. The liquid fraction containssolubilized proteins and lipids. While this washing step removes aportion of undesirable lipids, it also undesirably removes proteins,particularly sarcoplasmic proteins. The recovered protein-rich waterfraction then can be introduced downstream into the process for furtherprocessing the insoluble fraction from the washing step so that theproteins in the wash liquid soluble fraction can be recovered. Theground animal protein source is pulverized with water which also cancontain acid, such as citric acid to obtain a pH such as from about 5.3to about 5.5 to produce small particles which promote theirsolubilization in a subsequent step wherein the pH of the composition isreduced. When conducting this step at a pH between about 5.3 and about5.5, undesirable swelling of the composition is avoided or minimized.

[0044] The composition of pulverized protein-rich composition then ismixed with an acid composition to reduce the pH below about 3.5 but notso low as to significantly destroy the protein, such as about 2.0 oreven as low as about 1.0. Suitable acids are those which do notsignificantly destroy the protein and do not render the final producttoxic. Representative suitable acids include hydrochloric acid, sulfiricacid or the like. This process step conducted at low pH contrasts withthe prior art process conditions at a high pH close to neutral pH. Theresulting composition comprises a low viscosity solution in whichsubstantially all of the protein from the animal protein source issoluble and is substantially free of myofibrils and sarcomere tissuestructure.

[0045] The low pH solution then can be fractionated, if desired toseparate solids from the aqueous fraction or fractions, such as byfiltration or decantation to remove solids, such as bone, when present.The protein-rich aqueous component is recovered for further processingas described below.

[0046] The protein in the low viscosity solution then is treated toprecipitate the proteins. The protein in solution then is precipitatedsuch as by raising the solution pH above about 5.0, preferably to about5.5. Alternatively, salt or a precipitating polymer can be used toeffect precipitation. When the above-described washing step of theinitially ground tissue is eliminated, the water-soluble protein,including the sarcoplasmic protein from the ground tissue is recoveredin this step. Typically, the sarcoplasmic protein comprises about 20-30%of the total protein in the original tissue. The processes of the priorart do not recover this protein. While the initial washing step removesthis protein from the tissue being processed, it can be recovered in theprocess of this invention as described above. Even when this initialwashing step is included in the process of this invention and thesarcoplasmic protein is recovered separately, the process of thisinvention provides substantial advantages since it is capable ofprocessing animal protein sources, including high fat and high oilsources which can not be economically processed to produce food forhuman consumption with presently available processes.

[0047] The product of this invention differs from the products of theprior art in that the precipitated solid and liquid solution products ofthis invention are substantially free of myofibrils and sarcomeretissue. In contrast, the products of the prior art processes forproducing surimi contain myofibrils and sarcomeres. In addition, theproduct of this invention which comprises primarily myofibrillarprotein, also can contain significant amounts of sarcoplasmic protein.The sarcoplasmic protein in the protein product typically comprisesabove about 8%, preferably above about 15% and most preferably aboveabout 18% sarcoplasmic proteins by weight, up to about 30% by weightbased on the total weight of protein in the product.

[0048] The precipitated product can be used directly as a food source.Alternatively, the precipitated product can be further treated such asby removing a portion of the water in the product, by lyophilization,freezing, or heat drying. The resultant product can be in the form of asolution, a gel or a dry particulate product. The product is useful as afood grade composition for human consump-and has a wide variety of uses.The product can be used, for example, to form the major portion ofartificial crab meat or as a food additive such as a binding agent orthe like. In addition, the product can be used as an emulsifier, as athickening agent, as a foaming agent, as a gelling agent, as a waterbinding agent or the like, particularly in food products.

[0049]FIG. 1 illustrates the general process of this invention includingsome optional process steps. In a first step an animal muscle proteinsource 10 can optionally be introduced into a conventional cold press orcentrifugation or the like, step 12 wherein the feed, such as groundfish, is subjected to a pressure, to separate an aqueous liquidcontaining fats and oils 13 from solid tissue 15. The solid animaltissue 15 then is ground in step 20 to increase its surface area.Alternatively, steps 12 and 20 can be reversed. The ground tissue 28 ispulverized and its pH reduced with an aqueous acidic solution to about5.0 to about 5.5 in step 34. The aqueous composition 36 then is mixedwith acid in step 38 to reduce its pH to between about 3.0 and about3.5. The aqueous rich, protein containing streams can be added to step38 for processing therein. The resultant low pH protein rich fraction40, directed to step 58 wherein its pH is raised, such as to betweenabout 5.0 about 6.5 to effect precipitation of substantially all proteinin solution. Optionally, stream 56 can be treated such as by saltprecipitation, precipitation with a precipitating polymer orcombinations thereof or the like, rather than being precipitated in step58. The precipitated protein 60 can be further processed in step 62 suchas by lyophilization, freezing in the presence of a cryoprotectant or bybeing gelled.

[0050] The following example illustrates the present invention and arenot intended to limit the same.

EXAMPLE 1

[0051] This example provides a comparison of the process of thisinvention to a presently used process of the prior art.

[0052] The following is a description of a process developed toconcentrate, and extract proteins from muscle sources in a manner thatallows the proteins to retain their functionality (i.e., gelation,emulsification, etc.) throughout the process and into storage. The newacid solubilization/precipitation (ASP) preferred process of thisinvention is compared to the standard conventional procedure for surimimanufacture, as well as a recent improved conventional process. Theimproved conventional process was designed to produce a better gel withwhiter color and to remove more lipid than was obtained using theconventional method. Flow diagrams for the three processes are shown inFIGS. 2, 3 and 4. In all three procedures the initial steps, heading,gutting, the optional filleting, rinsing and mincing are performed usingstandard fish processing equipment. It is after these initial steps thatthe ASP process of this invention substantially changes from the othertwo processes. The goals of the conventional and improved conventionalprocesses are to keep the proteins under conditions which promote theirinsolubility, while washing away or removing undesirable solublecomponents. However, an undesirable sizable loss in protein results.Using the ASP process, conditions are adjusted to promote thesolubilization of all the muscle proteins. The conditions are a pH ofless than about 3.5 but not too low as to cause destruction to theproteins, and an ionic strength of less than or equal to about 200 mM.

[0053] Conventional Process

[0054] The basic steps of the conventional process are shown in FIG. 2.The amount of times or volumes in the wash steps can vary. Ground orminced fish is washed with refrigerated water (˜6° C.) long enough andin large enough volumes to remove undesirable components. Over-washingof the flesh can cause swelling of the proteins, which has been shown tointerfere with de-watering and to be deleterious to gel formation. Alarge proportion of the water soluble components are removed in thefirst wash with relatively less in subsequent washes. Time spent in thewash, or residence time, also determines washing effectiveness. Between9-12 minutes has been shown to be an adequate effective residence timeper wash. De-watering after each wash is accomplished using a rotaryscreen. This device is a continuous rolling screen with perforations ofapproximately 1 mm that allow a partial de-watering. Salt can be addedto the final wash to facilitate dewatering. After the final partialde-watering, the washed mince is passed through a refiner. In therefiner, the washed mince is forced against a screen with 0.5 mmperforations under high pressure from a concentric auger. Refining isreferred to as the “clean-up” step, where only finely minced muscle isallowed through the perforations. However, separation is not completeand some product is lost in this step. Diverted to a different locationis the refiner run-off, which consists of tiny bone and skin fragmentsand dark muscle, which tends to form in particles larger than 0.5 mm.The refiner is effective at removing unwanted non-edible fragments, butit is not 100% efficient and some particles do get through to the mince.The moisture content at this stage of production is approximately 90%.High moisture allows the refining process to function more effectively.To reduce the moisture content down to the desired 80% refined mince isplaced into a screw press. The screw press, like the refiner, pushes themince against a screen with 0.5 mm perforations using an augertransport, except that the screw press is under higher pressures.Cryoprotectants are added to the de-watered mince to protect theproteins from freeze denaturation and preserve their functionality. Acommon mixture of cryoprotectants is 4% sucrose, 4% sorbitol and 0.2%sodium tripolyphosphate. In the final step, product is frozen in a platefreezer, which rapidly freezes the product to guard against proteindenaturation as does occur during slow freezing.

[0055] Improved Conventional Process

[0056] Three main points of the improved conventional process (FIG. 3)differentiate the process from the conventional process. First, itimproves color (lightens) the product by using a “micronization” step,which decreases particle size down to 1-2 microns. This allows veryefficient leaching of the undesirable components out of the tissue dueto the large surface area. Second, the process also minces or micronizesthe tissue under vacuum (10 mm Hg) which has been shown to be effectiveat reducing oxidation of the lipids. The low vapor pressure caused bythe vacuum environment also promotes greater removal of low molecularweight compounds responsible for off or rancid odors. Third, the step inthe process which produces the most dramatic effect to the productsimprovement is that of the addition of sodium bicarbonate (0.1%) andsodium pyrophosphate (0.05-0.1%) to the first wash. The compoundsincrease the pH of the first wash to approximately 7.2-7.5, whichultimately causes an increase in the gels elasticity and reduces thelipid content to approximately 1%. The process, however, also increasesthe amount of protein lost during the leaching step. Due to themicronization step, product has to be recovered using centrifugation,which can recover the minute washed tissue particles. The remainingcryoprotection and freezing steps are similar to the conventionalprocess.

[0057] Acid Solubilization Precipitation (ASP) Process

[0058] As mentioned above, a preferred ASP process radically departsfrom the conventional and improved conventional process following thetissue disruption step. The whole tissue is homogenized in its dilutionmedium. The homogenization step places muscle tissue (ground or whole)into a solution of 1 mM citric acid, pH 3.0, preferably at a ratio of 1part tissue to 9 or more parts solution. Lower ratios of tissue solutioncan be employed depending upon the source of animal tissue in order toavoid gelation. Homogenization equipment that can be used is a PolytronKinematic homogenizer on speed 76 (1-2 min). The procedure can be scaledup using an Urshel Commitrol Model 1700 or comparable piece ofequipment. After homogenization, the pH of the resulting solution isapproximately 5.3 to 5.5. At this pH, which is near the isoelectricpoint of many of the muscle proteins, the up-take of solution by theproteins is at a minimum. This prevents hydration of the proteins andkeeps the viscosity low. The pH of the homogenate is then lowered toabout pH 3.5 or lower using, but not limited to, hydrochloric acid(HC1). 1 M HCl typically was used, but other mineral or organic acidscan perform equally well.

[0059] When a 1:9 ratio of tissue: to low pH (≦pH 3.5) solution is usedthen the resultant protein concentration will be approximately 16 mg/mlfor fish and 22 mg/ml for chicken. Viscosities for these solutions canvary from approximately 5 to 30 mpa·s depending on proteinconcentration. In virtually all muscle tissue examined using this low pH(and ionic strength) solubilization technique, solubility of theproteins has been between 90-100%.

[0060] At the stage in the process when the majority of proteins are insolution, processes such as heating (to destroy possible pathogens orenzymes), additive addition (antioxidants, polymer components, orprotein crosslinkers) and/or fractionation of the proteins by sizeexclusion chromatography or ultrafiltration can be performed. Also,since liquid media are much easier to handle than solids, transportingthe product with pumps can be done at this time.

[0061] In the next step, raising the pH to a point where the proteinsare least soluble and precipitate can be accomplished using numeroustypes of alkaline compounds. The pH was increased using 1M NaOH for acoarse adjustment and 100 mM NaOH for fine adjustment. Once the solutionis adjusted, the proteins can be visualized as white “threads” in thesolution. The threads start to appear at pH 3.8 and their concentrationsteadily increases as the pH increases. At pH values greater than adesired pH, depending upon the source of animal tissue, the solutionstarts to thicken and takes on a glossy appearance. Samples centrifugedat these higher pHs have large amounts (as high as 40%) of their proteinstay in the supernatant and are thus not recovered. Collecting theprotein is accomplished by centrifugation; however, the protein can alsobe obtained by filtration. The moisture content of the sedimentingprotein can be somewhat controlled by the centrifugation force. Acentrifugation force of 34,000×gravity produced Atlantic cod proteinwith 78% moisture, whereas, a force of 2575×gravity (table topcentrifuge) produced a sample with a moisture content of 84%. Salt orcharged polymers also can be used to effect precipitation.

[0062] Collected protein can be manufactured into a standard surimiproduct by the addition of cryoprotectants such as 4% sucrose, 4%sorbitol and 0.5% sodium tripolyphosphate and sufficient base such assodium carbonate and/or sodium hydroxide to obtain the desired pH from5.5 to approximately 7.0. The proteins with the cryoprotectants arefrozen in a plate freezer, which is standard in the industry.

[0063] A protein powder having a pH of about 3.0 is useful in themanufacture of increased protein beverages, such as is found in fruit orsport drinks. To lower the moisture content, it is possible toprecipitate the proteins at pH 5.5 and then re-acidify to pH 3.0 using,at most about one-tenth the original volume. This step was done usingAtlantic cod proteins, where the protein in solution was increased from1% to 6.1% prior to drying. This powder can also be used as anemulsifying agent in products such as mayonnaise or salad dressings.

[0064] Another product was produced by drying, under vacuum, theprecipitated protein from Atlantic cod to which cryoprotectants wereadded. The powder was hydrated to produce a gel with a strain of 1.1,stress of 26.6 kPa, and a whiteness index of 61.2. Visually the gelcontained small particles of tough tissue, which may have been areaswhere the proteins highly interacted with each other. The incorporationof low or high molecular weight agents, such as negatively chargedstarches, or sugars can improve the product by interfering withprotein-protein interactions. These compounds can be added to thesolution at low pH before precipitation.

[0065] Major Differences between Processes

[0066] 1. Yield Using the conventional process, protein recoveriesbetween 55-65% are commonly found using fish mince as the startingmaterial. Both myofibrillar and sarcoplasmic proteins are removed duringthe washing steps, with a large majority of these proteins beingsarcoplasmic. A large proportion of these proteins come off in the firstwash step. The improved conventional process loses additional proteindue to the increased pH in the first wash. Yields as low as 31% havebeen reported. In the ASP process of this invention, higher recoveriesof proteins are obtained. Typical protein recoveries using the ASPprocess are shown in Table 1. TABLE 1 Protein recoveries for differentspecies using the ASP process Muscle type Protein recovery (%) Chickenbreast 84, 92*, 94 Chicken thigh (dark) 76 Atlantic herring (light) 88Atlantic mackerel (light) 91 Atlantic cod 92 capelin (headed and gutted)63

[0067] 2. Gel Values It is generally believed that a strain value of 1.9is the minimum value needed to be obtained by a gel to be considered asa grade AA gel. The strain value is a measure of cohesiveness orelasticity, thought to be a desired attribute of an excellent gel. Table2 reports the strain along with the stress values for samplesmanufactured using the ASP process. For a comparison, a strain value of1.12 was obtained by using Atlantic mackerel surimi, manufactured usingthe conventional process on a semi-commercial scale at theNOAA-Mississippi State Univ. seafood pilot plant in Pascagoula, Miss.TABLE 2 Rheological values for samples manufactured using the ASPprocess Fish (quality) Strain Stress (kPa) Atlantic cod (v. good) 2.78 ±0.91 21.98 ± 2.02 Capelin (v. poor) 2.31 ± 0.22  45.04 ± 11.15 Atlanticmackerel light 2.61 ± 0.09 31.11 ± 3.82 (fair)

[0068] 3. Color Surimi from Atlantic cod produced using the ASP processdeveloped even whiter gels than surimi with Atlantic cod in theconventional process with a “L” value of 82.3, an “a” value of −0.11,and a “b” value of 2.88. The resultant whiteness index for this samplewas 82.1. Values of about 75 or higher are considered excellent.

[0069] 4. Advantages to Liquid Form The ASP process reduces animalmuscle tissue from a solid into a low viscosity fluid with substantiallyall the proteins in solution. From a processing point of view, thisprovides a great advantage. Liquids are easier to handle than solids. Amajor problem in the surimi industry is that bones, skin and blemishescontaminate the end-product. However, as a liquid, proteins in the ASPprocess can be centrifuged or filtered to assure no contamination entersthe final product. The use of the liquid protein solution alsosimplifies contaminant removal such as metal fragments from equipment.This is a major concern in the production of food. The liquid phase canalso be easily temperature controlled in operations such aspasteurization for the elimination of pathogens or rapid cooling.Equipment to move liquids is also much cheaper than equipment needed tomove solids. Having the proteins in a liquid form also facilitatesfractionating the proteins for either increasing or eliminating specificor groups of proteins. The ASP process also saves processing timebecause it eliminates the time needed for three or more washes as in theconventional process and can eliminate the refining step. Thesolubilization step of the proteins takes very little time and can beaccomplished in a one-pass system.

SUMMARY

[0070] The primary attributes of the process is that it permits thecomplete solubilization of substantially all of the muscle proteins intoa low viscosity fluid. The ASP process can be used to obtain high yieldsof washed minced fish and to regenerate the functional properties of themuscle proteins from aged or frozen samples. The ASP process allows theproteins obtained to be used in a wide array of food grade products andproduct enhancers since the products retain the protein

1. A process for forming a protein rich component of animal muscletissue which comprises forming a protein rich aqueous liquid solutionfrom a composition containing said tissue and an aqueous compositionhaving a pH less than about 3.5 which does not substantially degradeprotein of said protein rich component, and recovering said protein richcomponent.
 2. The process of claim 1 wherein said composition of saidanimal muscle tissue comprises a suspension of a particulate form of atissue in an aqueous solution having a pH between about 5.0 and about6.5.
 3. The process of any one of claims 1 or 2 wherein said proteinrich component is an aqueous liquid solution of protein derived frommuscle tissue having a pH less than about 3.5.
 4. The process of any oneof claims 1 or 2 wherein protein of said protein rich component insolution is recovered by precipitation.
 5. The process of claim 4wherein protein precipitation is effected by raising the pH of saidprotein rich component to between about 5.0 and 6.5.
 6. The process ofclaim 5 including the step of lyophilizing protein recovered from saidprecipitation step.
 7. The process of claim 5 including the step ofspray drying protein recovered from said precipitation step.
 8. Theprocess of claim 3 including the step of fractionating said proteinrecovered from said precipitation step.
 9. A protein rich solidcomposition isolated from an animal muscle tissue which comprisesmyofibrillar proteins substantially free of myofibrils and sarcomeres.10. The composition of claim 9 which contains at least about 8% up toabout 30% by weight sarcoplasmic proteins based upon total weight ofprotein.
 11. The composition of claim 9 which contains at least about10% up to about 30% by weight sarcoplasmic proteins based upon totalweight of protein.
 12. The composition of claim 9 which contains atleast about 15% up to about 30% by weight sarcoplasmic proteins basedupon total weight of proteins.
 13. The composition of claim 9 whichcontains at least about 18% up to about 30% by weight sarcoplasmicproteins based upon total weight of protein.
 14. A protein richcomposition isolated from animal muscle tissue comprising myofibrillarproteins substantially free of myofibrils and sarcomeres in aqueousliquid solution having a pH less than about 3.5 which does notsubstantially degrade said proteins.
 15. The composition of claim 14having a pH between about 2.5 and about 3.5.
 16. The composition of anyone of claims 14 or 15 which contains at least about 8% up to about 30%by weight sarcoplasmic proteins based on total weight of myofibrillarprotein and sarcoplasmic protein.
 17. The composition of any one ofclaims 14 or 15 which contains at least about 10% up to about 30% byweight sarcoplasmic proteins based on total weight of myofibrillarprotein and sarcoplasmic protein.
 18. The composition of any one ofclaims 14 or 15 which contains at least about 15% by weight sarcoplasmicproteins based on total weight of myofibrillar protein and sarcoplasmicprotein.
 19. The composition of any one of claims 14 or 15 whichcontains at least about 18% by weight sarcoplasmic proteins based ontotal weight of myofibrillar protein and sarcoplasmic protein.
 20. Theprocess of claim 1 wherein said pH is between about 2.5 and about 3.5.21. The process of any one of claims 1 or 2 wherein said animal muscletissue is fish tissue.
 22. The process of any one of claims 1 or 2wherein said animal muscle tissue is chicken tissue.
 23. The process ofclaim 5 wherein said pH is raised with a composition that includes apolyphosphate.
 24. The process of any one of claims 1 and 2 wherein saidaqueous liquid composition having a pH less than about 3.5 is formedwith citric acid.